In many brain regions, glial cells substantially outnumber nerve cells, but their role in physiological and pathophysiological conditions remains poorly understood. Astrocytes are the most widely distributed glia with intimate anatomical interactions with excitatory synapses. Recent studies reveal that, in addition to providing structural and nutritional support, astrocytes dictate synapse formation and subsequent synapse refinement- elimination in the developing CNS. Our preliminary results show that, after chronic exposure to cocaine or morphine, some of these glia-based developmental mechanisms re-emerge in the adult nucleus accumbens (NAc), a forebrain region essential for addiction-related behavioral abnormalities. These drug-induced, glia- mediated synaptic remodeling processes may profoundly rewire the neurocircuits involving the NAc, and critically contribute to the pathophysiology of drug addiction. Focusing on this unique angle, the objectives of this application are: 1) To characterize the molecular and cellular mechanisms underlying glia-mediated synaptogenesis and synaptodegeneration in the NAc in mice after cocaine or morphine self-administration and withdrawal; 2) To determine the circuitry consequences of drug-induced, glia-mediated synaptic remodeling, particularly, how NAc excitatory synapses are refashioned in cocaine- and morphine-exposed mice by glia- mediated synaptogenesis or synaptodegeneration; and 3) To determine the behavioral consequences of drug- induced, glia-mediated synapse and circuitry remodeling using the mouse model of incubation of cue-induced drug craving, a drug relapse model that depends on NAc excitatory circuits. To achieve these goals, we will use a multidisciplinary approach, across the Dong and Nestler laboratories, including confocal imaging, slice electrophysiology, optogenetics, in vivo viral-mediated gene transfer, RNA interference, transgenic mouse lines, and mouse models of drug self-administration. By targeting the previously unexplored glia-mediated synapse and circuitry remodeling in drug-exposed mice, the proposed experiments promise to open new avenues toward understanding cellular and circuitry mechanisms underlying drug addiction and providing new strategies for anti-addiction treatments.